Study Of Controlled Leaching Of The Single-Well Retreating Horizontal Cavern For Energy Storage In Deep Salt Rock Formations

Salt rock energy storage is necessary to achieve clean energy self-peaking and industrial applications due to the obvious advantages of high working gas volumes,low gas cushion volumes and high safety.Salt rock energy storage in China will develop in deeper salt rock formations with the increase of construction capacity.However,significant differences exist in the composition and properties of deep and shallow salt rocks due to the different formation and deposit conditions,which pose problems and challenges for constructing deep salt rock energy storage.Although there are differences in burial depth,the sedimentary characteristics of salt rock deposits in China are lacustrine-layered salt rock deposits,with thin single-layer salt rock layers,many interlayers,and frequent interaction between salt rock layers and interlayers.For the lacustrine-layered salt rock deposits,the single-well retreating horizontal leaching method combines all the advantages of the two-well convection horizontal and the single-well convection vertical leaching methods.The single-well retreating horizontal leaching method has higher leaching efficiency and better adaptability to lacustrine-layered salt rock deposits than the single-well convection vertical leaching method.At the same time,the single-well retreating horizontal leaching method can effectively control the cavern expansion while efficiently using salt rock formation compared with the two-well convection horizontal leaching method,thus reducing the safety risk.However,the cavern expansion and key parameters during the single-well retreating horizontal leaching process are still unclear.At the same time,the stability of the single-well retreating horizontal salt rock storage in deep salt rock formations needs to be verified.This study mainly focused on the single-well retreating horizontal leaching method.The main research methods included rock physics and mechanics tests,indoor physical similarity simulation,theoretical analysis and numerical simulation.The physical and mechanical properties of deep salt rocks in the Liulin mining area with a burial depth of over 1900 m were measured.The cavern expansion and the key parameters influence during the single-well retreating horizontal leaching process were investigated,a leaching control scheme for the dissolution construction of a single-well retreating horizontal cavern in the Liulin mining area was given,and the stability and reliability of horizontal storage in deep salt rock formations were analysed.The main research contents and conclusions of this paper were summarised as follows:（1）Uniaxial compression,triaxial compression,Brazilian splitting and variable angle shear mechanical tests were carried out on the deep salt rocks in the Liulin mining area.The mechanical parameters of the deep salt rocks in the Liulin mining area were determined.The differences in the physical and mechanical properties of the salt rocks between Liulin and other mining areas in China were compared.The results showed that the average uniaxial compressive strength,Yang’s modulus,Poisson’s ratio,and tensile strength of the salt rocks in the Liulin mining area were 19.53 MPa,3.69 GPa,0.29,and 1.65 MPa.The angle of internal friction was 33.75°,and the cohesion was 6.94 MPa.The lower insoluble matter content and higher tensile strength of the salt rocks in the Liulin mining area were more conducive to the economics of the energy storage leaching process and the stability of the operational process.（2）Triaxial creep mechanical tests determined the creep mechanical properties of deep salt rocks in the Liulin mining area under different temperature conditions.The parameters of the steady-state creep instanton equation considering temperature and deviatoric stress were fitted by regression:creep stress index m=4.83,activation free energy Q=7.84×10⁴ k J·mol^-1,and material parameter A=32.39 MPa^-4.83·h^-1.A fractional-order viscoelastic-plastic creep damage model was constructed by introducing fractional-order viscous pot elements to describe the initial,steady-state and accelerated creep processes of deep salt rocks,considering the effects of temperature and deviatoric stress.A comparison of the creep mechanical properties of salt rocks in China showed that the creep rate of Liulin salt rocks was lower under the same external environment.（3）The single-well retreating horizontal leaching process was studied through indoor physical similarity tests,and the cavern expansion rule was studied.The influence of water injection direction and water injection rate on the leaching efficiency was analysed,and the influence of the air cushion on the cavern expansion and the brine concentration distribution was explored.The results showed that the leaching efficiency increased 5.36 times after the water injection rate increased 8.0 times.The leaching efficiency of upward water injection was about 1.24 times that of horizontal water injection along the pipe column at the same water injection rate.The dissolution rate of salt rock near the gas-liquid interface was 5.41 times higher than that before air injection,and the brine concentration near the gas-liquid interface was only 32.97%～69.86%of that before air injection.（4）A multi-field coupled thermal-flow-mass transfer-boundary movement numerical model was established based on clarifying the influence of key parameters in the single-well retreating horizontal leaching process to analyse the interaction characteristics between the brine temperature field,flow field,concentration field and cavern boundary expansion in the single-well retreating horizontal leaching process.Given the Liulin geological characteristics,leaching efficiency,formation utilisation rate,cavern shape regularity coefficient and water-saving coefficient were adopted as evaluation indexes.The two leaching control schemes were compared:Scheme 1,which ensured that the outlet brine concentration reached the acceptable standard as far as possible.Scheme 2,which increased the leaching efficiency as far as possible.Scheme 1 was adopted for the single-well retreating horizontal leaching process in the Liulin mining area following the principles of safety,economy,and efficiency.（5）Based on the measured mechanical parameters of deep salt rocks and the single-well retreating horizontal cavern shape in the Liulin mining area,a numerical model was established by the secondary development of the fractional-order viscoelastic-plastic creep damage model for stability analysis of deep salt rock gas storage.The maximum and minimum internal gas pressure and injection-extraction cycle patterns were optimised,and the non-linear response characteristics during uncontrolled blowout were analysed.The recommended maximum and minimum internal gas pressures were 40 MPa and 26 MPa,respectively.An appropriate increase in the frequency of internal gas circulation better served the stability.The increased rates of displacement and volume loss rate during uncontrolled blowout were tens of times faster than during operation,and the plastic zone volume increased by 154.32%.